Whole-brain snapshot CEST imaging at 7 T using 3D-EPI

Abstract

Purpose: The aim of this work is to develop a fast and robust CEST sequence in order to allow the acquisition of a whole-brain imaging volume after a single preparation block (snapshot acquisition).

Methods: A 3D-CEST sequence with an optimized 3D-EPI readout module was developed, which acquires the complete k-space data following a single CEST preparation for 1 saturation offset. Whole-brain mapping of the Z-spectrum with 2 mm isotropic resolution is achieved at 68 saturation frequencies in 5 minutes (4.33 s per offset). We analyzed the $B_1$ distribution in order to optimize $B_1$ correction and to provide accurate CEST quantification across the whole brain.

Results: We obtained maps for 3 different CEST contrasts from 4 healthy subjects. Based on our $B_1$ distribution analysis, we conclude that 3 $B_1$ sampling points allow for sufficient compensation of $B_1$ variations across most of the brain. Two brain regions, the cerebellum and the temporal lobes, are difficult to quantify at 7 T due to very low $B_1$ that was achieved in these regions.

Conclusions: The proposed sequence enables robust acquisition of 2 mm isotropic whole-brain CEST maps at 7 Tesla within a total scan time of 16 minutes.

Keywords: $B_1$ -correction; 3D-EPI; APT; CEST; UHF; chemical exchange saturation transfer; rNOE; whole brain.